Method and apparatus for performing partial handover procedure in wireless communication system

ABSTRACT

A method and apparatus for performing a partial handover procedure in a wireless communication system is provided. An eNodeB (eNB) of a 3rd generation partnership project (3GPP) long term evolution (LTE) system determines E-UTRAN radio access bearers (E-RABs) to be offloaded to an access point (AP) of a wireless local area network (WLAN) system, and transmits an E-RAB release indication message, which includes a list of the E-RABs to be offloaded to the AP and a corresponding cause which is set to “Offload to WLAN”, to a mobility management entity (MME) of the 3GPP LTE system.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to wireless communications, and moreparticularly, to a method and apparatus for performing a partialhandover procedure in a wireless communication system.

2. Related Art

Universal mobile telecommunications system (UMTS) is a 3rd generation(3G) asynchronous mobile communication system operating in wideband codedivision multiple access (WCDMA) based on European systems, globalsystem for mobile communications (GSM) and general packet radio services(GPRS). The long-term evolution (LTE) of UMTS is under discussion by the3rd generation partnership project (3GPP) that standardized UMTS.

The 3GPP LTE is a technology for enabling high-speed packetcommunications. Many schemes have been proposed for the LTE objectiveincluding those that aim to reduce user and provider costs, improveservice quality, and expand and improve coverage and system capacity.The 3GPP LTE requires reduced cost per bit, increased serviceavailability, flexible use of a frequency band, a simple structure, anopen interface, and adequate power consumption of a terminal as anupper-level requirement.

FIG. 1 shows LTE system architecture. The communication network iswidely deployed to provide a variety of communication services such asvoice over internet protocol (VoIP) through IMS and packet data.

Referring to FIG. 1, the LTE system architecture includes one or moreuser equipment (UE; 10), an evolved-UMTS terrestrial radio accessnetwork (E-UTRAN) and an evolved packet core (EPC). The UE 10 refers toa communication equipment carried by a user. The UE 10 may be fixed ormobile, and may be referred to as another terminology, such as a mobilestation (MS), a user terminal (UT), a subscriber station (SS), awireless device, etc.

The E-UTRAN includes one or more evolved node-B (eNB) 20, and aplurality of UEs may be located in one cell. The eNB 20 provides an endpoint of a control plane and a user plane to the UE 10. The eNB 20 isgenerally a fixed station that communicates with the UE 10 and may bereferred to as another terminology, such as a base station (BS), a basetransceiver system (BTS), an access point, etc. One eNB 20 may bedeployed per cell. There are one or more cells within the coverage ofthe eNB 20. A single cell is configured to have one of bandwidthsselected from 1.25, 2.5, 5, 10, and 20 MHz, etc., and provides downlinkor uplink transmission services to several UEs. In this case, differentcells can be configured to provide different bandwidths.

Hereinafter, a downlink (DL) denotes communication from the eNB 20 tothe UE 10, and an uplink (UL) denotes communication from the UE 10 tothe eNB 20. In the DL, a transmitter may be a part of the eNB 20, and areceiver may be a part of the UE 10. In the UL, the transmitter may be apart of the UE 10, and the receiver may be a part of the eNB 20.

The EPC includes a mobility management entity (MME) which is in chargeof control plane functions, and a system architecture evolution (SAE)gateway (S-GW) which is in charge of user plane functions. The MME/S-GW30 may be positioned at the end of the network and connected to anexternal network. The MME has UE access information or UE capabilityinformation, and such information may be primarily used in UE mobilitymanagement. The S-GW is a gateway of which an endpoint is an E-UTRAN.The MME/S-GW 30 provides an end point of a session and mobilitymanagement function for the UE 10. The EPC may further include a packetdata network (PDN) gateway (PDN-GW). The PDN-GW is a gateway of which anendpoint is a PDN.

The MME provides various functions including non-access stratum (NAS)signaling to eNBs 20, NAS signaling security, access stratum (AS)security control, Inter core network (CN) node signaling for mobilitybetween 3GPP access networks, idle mode UE reachability (includingcontrol and execution of paging retransmission), tracking area listmanagement (for UE in idle and active mode), P-GW and S-GW selection,MME selection for handovers with MME change, serving GPRS support node(SGSN) selection for handovers to 2G or 3G 3GPP access networks,roaming, authentication, bearer management functions including dedicatedbearer establishment, support for public warning system (PWS) (whichincludes earthquake and tsunami warning system (ETWS) and commercialmobile alert system (CMAS)) message transmission. The S-GW host providesassorted functions including per-user based packet filtering (by e.g.,deep packet inspection), lawful interception, UE Internet protocol (IP)address allocation, transport level packet marking in the DL, UL and DLservice level charging, gating and rate enforcement, DL rate enforcementbased on APN-AMBR. For clarity MME/S-GW 30 will be referred to hereinsimply as a “gateway,” but it is understood that this entity includesboth the MME and S-GW.

Interfaces for transmitting user traffic or control traffic may be used.The UE 10 and the eNB 20 are connected by means of a Uu interface. TheeNBs 20 are interconnected by means of an X2 interface. Neighboring eNBsmay have a meshed network structure that has the X2 interface. The eNBs20 are connected to the EPC by means of an S1 interface. The eNBs 20 areconnected to the MME by means of an S1-MME interface, and are connectedto the S-GW by means of S1-U interface. The S1 interface supports amany-to-many relation between the eNB 20 and the MME/S-GW.

The eNB 20 may perform functions of selection for gateway 30, routingtoward the gateway 30 during a radio resource control (RRC) activation,scheduling and transmitting of paging messages, scheduling andtransmitting of broadcast channel (BCH) information, dynamic allocationof resources to the UEs 10 in both UL and DL, configuration andprovisioning of eNB measurements, radio bearer control, radio admissioncontrol (RAC), and connection mobility control in LTE ACTIVE state. Inthe EPC, and as noted above, gateway 30 may perform functions of pagingorigination, LTE_IDLE state management, ciphering of the user plane, SAEbearer control, and ciphering and integrity protection of NAS signaling.

FIG. 2 shows a control plane of a radio interface protocol of an LTEsystem. FIG. 3 shows a user plane of a radio interface protocol of anLTE system.

Layers of a radio interface protocol between the UE and the E-UTRAN maybe classified into a first layer (L1), a second layer (L2), and a thirdlayer (L3) based on the lower three layers of the open systeminterconnection (OSI) model that is well-known in the communicationsystem. The radio interface protocol between the UE and the E-UTRAN maybe horizontally divided into a physical layer, a data link layer, and anetwork layer, and may be vertically divided into a control plane(C-plane) which is a protocol stack for control signal transmission anda user plane (U-plane) which is a protocol stack for data informationtransmission. The layers of the radio interface protocol exist in pairsat the UE and the E-UTRAN, and are in charge of data transmission of theUu interface.

A physical (PHY) layer belongs to the L1. The PHY layer provides ahigher layer with an information transfer service through a physicalchannel. The PHY layer is connected to a medium access control (MAC)layer, which is a higher layer of the PHY layer, through a transportchannel. A physical channel is mapped to the transport channel. Data istransferred between the MAC layer and the PHY layer through thetransport channel. Between different PHY layers, i.e., a PHY layer of atransmitter and a PHY layer of a receiver, data is transferred throughthe physical channel using radio resources. The physical channel ismodulated using an orthogonal frequency division multiplexing (OFDM)scheme, and utilizes time and frequency as a radio resource.

The PHY layer uses several physical control channels. A physicaldownlink control channel (PDCCH) reports to a UE about resourceallocation of a paging channel (PCH) and a downlink shared channel(DL-SCH), and hybrid automatic repeat request (HARQ) information relatedto the DL-SCH. The PDCCH may carry a UL grant for reporting to the UEabout resource allocation of UL transmission. A physical control formatindicator channel (PCFICH) reports the number of OFDM symbols used forPDCCHs to the UE, and is transmitted in every subframe. A physicalhybrid ARQ indicator channel (PHICH) carries an HARQ acknowledgement(ACK)/non-acknowledgement (NACK) signal in response to UL transmission.A physical uplink control channel (PUCCH) carries UL control informationsuch as HARQ ACK/NACK for DL transmission, scheduling request, and CQI.A physical uplink shared channel (PUSCH) carries a UL-uplink sharedchannel (SCH).

FIG. 4 shows an example of a physical channel structure.

A physical channel consists of a plurality of subframes in time domainand a plurality of subcarriers in frequency domain. One subframeconsists of a plurality of symbols in the time domain. One subframeconsists of a plurality of resource blocks (RBs). One RB consists of aplurality of symbols and a plurality of subcarriers. In addition, eachsubframe may use specific subcarriers of specific symbols of acorresponding subframe for a PDCCH. For example, a first symbol of thesubframe may be used for the PDCCH. The PDCCH carries dynamic allocatedresources, such as a physical resource block (PRB) and modulation andcoding scheme (MCS). A transmission time interval (TTI) which is a unittime for data transmission may be equal to a length of one subframe. Thelength of one subframe may be 1 ms.

The transport channel is classified into a common transport channel anda dedicated transport channel according to whether the channel is sharedor not. A DL transport channel for transmitting data from the network tothe UE includes a broadcast channel (BCH) for transmitting systeminformation, a paging channel (PCH) for transmitting a paging message, aDL-SCH for transmitting user traffic or control signals, etc. The DL-SCHsupports HARQ, dynamic link adaptation by varying the modulation, codingand transmit power, and both dynamic and semi-static resourceallocation. The DL-SCH also may enable broadcast in the entire cell andthe use of beamforming. The system information carries one or moresystem information blocks. All system information blocks may betransmitted with the same periodicity. Traffic or control signals of amultimedia broadcast/multicast service (MBMS) may be transmitted throughthe DL-SCH or a multicast channel (MCH).

A UL transport channel for transmitting data from the UE to the networkincludes a random access channel (RACH) for transmitting an initialcontrol message, a UL-SCH for transmitting user traffic or controlsignals, etc. The UL-SCH supports HARQ and dynamic link adaptation byvarying the transmit power and potentially modulation and coding. TheUL-SCH also may enable the use of beamforming. The RACH is normally usedfor initial access to a cell.

A MAC layer belongs to the L2. The MAC layer provides services to aradio link control (RLC) layer, which is a higher layer of the MAClayer, via a logical channel. The MAC layer provides a function ofmapping multiple logical channels to multiple transport channels. TheMAC layer also provides a function of logical channel multiplexing bymapping multiple logical channels to a single transport channel. A MACsublayer provides data transfer services on logical channels.

The logical channels are classified into control channels fortransferring control plane information and traffic channels fortransferring user plane information, according to a type of transmittedinformation. That is, a set of logical channel types is defined fordifferent data transfer services offered by the MAC layer. The logicalchannels are located above the transport channel, and are mapped to thetransport channels.

The control channels are used for transfer of control plane informationonly. The control channels provided by the MAC layer include a broadcastcontrol channel (BCCH), a paging control channel (PCCH), a commoncontrol channel (CCCH), a multicast control channel (MCCH) and adedicated control channel (DCCH). The BCCH is a downlink channel forbroadcasting system control information. The PCCH is a downlink channelthat transfers paging information and is used when the network does notknow the location cell of a UE. The CCCH is used by UEs having no RRCconnection with the network. The MCCH is a point-to-multipoint downlinkchannel used for transmitting MBMS control information from the networkto a UE. The DCCH is a point-to-point bi-directional channel used by UEshaving an RRC connection that transmits dedicated control informationbetween a UE and the network.

Traffic channels are used for the transfer of user plane informationonly. The traffic channels provided by the MAC layer include a dedicatedtraffic channel (DTCH) and a multicast traffic channel (MTCH). The DTCHis a point-to-point channel, dedicated to one UE for the transfer ofuser information and can exist in both uplink and downlink. The MTCH isa point-to-multipoint downlink channel for transmitting traffic datafrom the network to the UE.

Uplink connections between logical channels and transport channelsinclude the DCCH that can be mapped to the UL-SCH, the DTCH that can bemapped to the UL-SCH and the CCCH that can be mapped to the UL-SCH.Downlink connections between logical channels and transport channelsinclude the BCCH that can be mapped to the BCH or DL-SCH, the PCCH thatcan be mapped to the PCH, the DCCH that can be mapped to the DL-SCH, andthe DTCH that can be mapped to the DL-SCH, the MCCH that can be mappedto the MCH, and the MTCH that can be mapped to the MCH.

An RLC layer belongs to the L2. The RLC layer provides a function ofadjusting a size of data, so as to be suitable for a lower layer totransmit the data, by concatenating and segmenting the data receivedfrom a higher layer in a radio section. In addition, to ensure a varietyof quality of service (QoS) required by a radio bearer (RB), the RLClayer provides three operation modes, i.e., a transparent mode (TM), anunacknowledged mode (UM), and an acknowledged mode (AM). The AM RLCprovides a retransmission function through an automatic repeat request(ARQ) for reliable data transmission. Meanwhile, a function of the RLClayer may be implemented with a functional block inside the MAC layer.In this case, the RLC layer may not exist.

A packet data convergence protocol (PDCP) layer belongs to the L2. ThePDCP layer provides a function of header compression function thatreduces unnecessary control information such that data being transmittedby employing IP packets, such as IPv4 or IPv6, can be efficientlytransmitted over a radio interface that has a relatively smallbandwidth. The header compression increases transmission efficiency inthe radio section by transmitting only necessary information in a headerof the data. In addition, the PDCP layer provides a function ofsecurity. The function of security includes ciphering which preventsinspection of third parties, and integrity protection which preventsdata manipulation of third parties.

A radio resource control (RRC) layer belongs to the L3. The RLC layer islocated at the lowest portion of the L3, and is only defined in thecontrol plane. The RRC layer takes a role of controlling a radioresource between the UE and the network. For this, the UE and thenetwork exchange an RRC message through the RRC layer. The RRC layercontrols logical channels, transport channels, and physical channels inrelation to the configuration, reconfiguration, and release of RBs. AnRB is a logical path provided by the L1 and L2 for data delivery betweenthe UE and the network. That is, the RB signifies a service provided theL2 for data transmission between the UE and E-UTRAN. The configurationof the RB implies a process for specifying a radio protocol layer andchannel properties to provide a particular service and for determiningrespective detailed parameters and operations. The RB is classified intotwo types, i.e., a signaling RB (SRB) and a data RB (DRB). The SRB isused as a path for transmitting an RRC message in the control plane. TheDRB is used as a path for transmitting user data in the user plane.

Referring to FIG. 2, the RLC and MAC layers (terminated in the eNB onthe network side) may perform functions such as scheduling, automaticrepeat request (ARQ), and hybrid automatic repeat request (HARQ). TheRRC layer (terminated in the eNB on the network side) may performfunctions such as broadcasting, paging, RRC connection management, RBcontrol, mobility functions, and UE measurement reporting andcontrolling. The NAS control protocol (terminated in the MME of gatewayon the network side) may perform functions such as a SAE bearermanagement, authentication, LTE_IDLE mobility handling, pagingorigination in LTE_IDLE, and security control for the signaling betweenthe gateway and UE.

Referring to FIG. 3, the RLC and MAC layers (terminated in the eNB onthe network side) may perform the same functions for the control plane.The PDCP layer (terminated in the eNB on the network side) may performthe user plane functions such as header compression, integrityprotection, and ciphering.

An RRC state indicates whether an RRC layer of the UE is logicallyconnected to an RRC layer of the E-UTRAN. The RRC state may be dividedinto two different states such as an RRC connected state and an RRC idlestate. When an RRC connection is established between the RRC layer ofthe UE and the RRC layer of the E-UTRAN, the UE is in RRC_CONNECTED, andotherwise the UE is in RRC_IDLE. Since the UE in RRC_CONNECTED has theRRC connection established with the E-UTRAN, the E-UTRAN may recognizethe existence of the UE in RRC_CONNECTED and may effectively control theUE. Meanwhile, the UE in RRC_IDLE may not be recognized by the E-UTRAN,and a CN manages the UE in unit of a TA which is a larger area than acell. That is, only the existence of the UE in RRC_IDLE is recognized inunit of a large area, and the UE must transition to RRC_CONNECTED toreceive a typical mobile communication service such as voice or datacommunication.

In RRC_IDLE state, the UE may receive broadcasts of system informationand paging information while the UE specifies a discontinuous reception(DRX) configured by NAS, and the UE has been allocated an identification(ID) which uniquely identifies the UE in a tracking area and may performpublic land mobile network (PLMN) selection and cell re-selection. Also,in RRC_IDLE state, no RRC context is stored in the eNB.

In RRC_CONNECTED state, the UE has an E-UTRAN RRC connection and acontext in the E-UTRAN, such that transmitting and/or receiving datato/from the eNB becomes possible. Also, the UE can report channelquality information and feedback information to the eNB. InRRC_CONNECTED state, the E-UTRAN knows the cell to which the UE belongs.Therefore, the network can transmit and/or receive data to/from UE, thenetwork can control mobility (handover and inter-radio accesstechnologies (RAT) cell change order to GSM EDGE radio access network(GERAN) with network assisted cell change (NACC)) of the UE, and thenetwork can perform cell measurements for a neighboring cell.

In RRC_IDLE state, the UE specifies the paging DRX cycle. Specifically,the UE monitors a paging signal at a specific paging occasion of everyUE specific paging DRX cycle. The paging occasion is a time intervalduring which a paging signal is transmitted. The UE has its own pagingoccasion.

A paging message is transmitted over all cells belonging to the sametracking area. If the UE moves from one TA to another TA, the UE willsend a tracking area update (TAU) message to the network to update itslocation.

When the user initially powers on the UE, the UE first searches for aproper cell and then remains in RRC_IDLE in the cell. When there is aneed to establish an RRC connection, the UE which remains in RRC_IDLEestablishes the RRC connection with the RRC of the E-UTRAN through anRRC connection procedure and then may transition to RRC_CONNECTED. TheUE which remains in RRC_IDLE may need to establish the RRC connectionwith the E-UTRAN when uplink data transmission is necessary due to auser's call attempt or the like or when there is a need to transmit aresponse message upon receiving a paging message from the E-UTRAN.

It is known that different cause values may be mapped o the signaturesequence used to transmit messages between a UE and eNB and that eitherchannel quality indicator (CQI) or path loss and cause or message sizeare candidates for inclusion in the initial preamble.

When a UE wishes to access the network and determines a message to betransmitted, the message may be linked to a purpose and a cause valuemay be determined. The size of the ideal message may be also bedetermined by identifying all optional information and differentalternative sizes, such as by removing optional information, or analternative scheduling request message may be used.

The UE acquires necessary information for the transmission of thepreamble, UL interference, pilot transmit power and requiredsignal-to-noise ratio (SNR) for the preamble detection at the receiveror combinations thereof. This information must allow the calculation ofthe initial transmit power of the preamble. It is beneficial to transmitthe UL message in the vicinity of the preamble from a frequency point ofview in order to ensure that the same channel is used for thetransmission of the message.

The UE should take into account the UL interference and the UL path lossin order to ensure that the network receives the preamble with a minimumSNR. The UL interference can be determined only in the eNB, andtherefore, must be broadcast by the eNB and received by the UE prior tothe transmission of the preamble. The UL path loss can be considered tobe similar to the DL path loss and can be estimated by the UE from thereceived RX signal strength when the transmit power of some pilotsequence of the cell is known to the UE.

The required UL SNR for the detection of the preamble would typicallydepend on the eNB configuration, such as a number of Rx antennas andreceiver performance. There may be advantages to transmit the ratherstatic transmit power of the pilot and the necessary UL SNR separatelyfrom the varying UL interference and possibly the power offset requiredbetween the preamble and the message.

The initial transmission power of the preamble can be roughly calculatedaccording to the following formula:

Transmit power=TransmitPilot−RxPilot+ULInterference+Offset+SNRRequired

Therefore, any combination of SNRRequired, ULlnterference, TransmitPilotand Offset can be broadcast. In principle, only one value must bebroadcast. This is essentially in current UMTS systems, although the ULinterference in 3GPP LTE will mainly be neighboring cell interferencethat is probably more constant than in UMTS system.

The UE determines the initial UL transit power for the transmission ofthe preamble as explained above. The receiver in the eNB is able toestimate the absolute received power as well as the relative receivedpower compared to the interference in the cell. The eNB will consider apreamble detected if the received signal power compared to theinterference is above an eNB known threshold.

The UE performs power ramping in order to ensure that a UE can bedetected even if the initially estimated transmission power of thepreamble is not adequate. Another preamble will most likely betransmitted if no ACK or NACK is received by the UE before the nextrandom access attempt. The transmit power of the preamble can beincreased, and/or the preamble can be transmitted on a different ULfrequency in order to increase the probability of detection. Therefore,the actual transmit power of the preamble that will be detected does notnecessarily correspond to the initial transmit power of the preamble asinitially calculated by the UE.

The UE must determine the possible UL transport format. The transportformat, which may include MCS and a number of resource blocks thatshould be used by the UE, depends mainly on two parameters, specificallythe SNR at the eNB and the required size of the message to betransmitted.

In practice, a maximum UE message size, or payload, and a requiredminimum SNR correspond to each transport format. In UMTS, the UEdetermines before the transmission of the preamble whether a transportformat can be chosen for the transmission according to the estimatedinitial preamble transmit power, the required offset between preambleand the transport block, the maximum allowed or available UE transmitpower, a fixed offset and additional margin. The preamble in UMTS neednot contain any information regarding the transport format selected bythe EU since the network does not need to reserve time and frequencyresources and, therefore, the transport format is indicated togetherwith the transmitted message.

The eNB must be aware of the size of the message that the UE intends totransmit and the SNR achievable by the UE in order to select the correcttransport format upon reception of the preamble and then reserve thenecessary time and frequency resources. Therefore, the eNB cannotestimate the SNR achievable by the EU according to the received preamblebecause the UE transmit power compared to the maximum allowed orpossible UE transmit power is not known to the eNB, given that the UEwill most likely consider the measured path loss in the DL or someequivalent measure for the determination of the initial preambletransmission power.

The eNB could calculate a difference between the path loss estimated inthe DL compared and the path loss of the UL. However, this calculationis not possible if power ramping is used and the UE transmit power forthe preamble does not correspond to the initially calculated UE transmitpower. Furthermore, the precision of the actual UE transmit power andthe transmit power at which the UE is intended to transmit is very low.Therefore, it has been proposed to code the path loss or CQI estimationof the downlink and the message size or the cause value In the UL in thesignature.

In a cellular system such as 3GPP system, methods for interworking ofthe 3GPP system and wireless local area network (WLAN) system has beendiscussed in order to use limited radio resources effectively. Amongvarious methods for 3GPP-WLAN interworking, a method for offloadingtraffic of the 3GPP system to the WLAN system has been mainly discussed.By data offloading from the 3GPP system to the WLAN system, radioresources allocated to the 3GPP system and the WLAN system can be usedeffectively. Data offloading may be called another terminology, e.g.,traffic steering.

3GPP-WLAN interworking and integration is currently supported by the3GPP system at the core network (CN) level, including both seamless andnon-seamless mobility to the WLAN system. However, as operatorcontrolled WLAN deployments become more common and WLAN usage increases,RAN level enhancements for WLAN interworking which may improve userexperience, provide more operator control and better access networkutilization and reduced operational expenditure (OPEX) may be needed.Currently, it has been agreed to study potential radio access network(RAN) level enhancements for 3GPP-WLAN interworking in 3GPP LTE rel-12.The followings are issues which should be taken into account for3GPP-WLAN interworking.

-   -   Operator deployed WLAN networks are often under-utilized;    -   User experience is suboptimal when UE connects to an overloaded        WLAN network;    -   Unnecessary WLAN scanning may drain UE battery resources.

Discussion for 3GPP-WLAN interworking may be divided in two phases. Inthe first phase, identifying the requirements for RAN level interworkingand clarifying the scenarios to be considered while taking into accountexisting standardized mechanisms may be discussed. In the second phase,identifying solutions addressing the requirements identified in thefirst phase which cannot be solved using existing standardizedmechanisms may be discussed.

The following may be assumed for 3GPP-WLAN interworking.

-   -   There is no need to distinguish between indoor and outdoor        deployment scenarios.    -   Solutions developed should not rely on standardized interface        between 3GPP and WLAN RAN nodes.

The candidate solutions to be considered should meet the followingrequirements.

-   -   Solutions should provide improved load balancing between WLAN        and 3GPP radio access networks in order to provide improved        system capacity.    -   Solutions should improve performance (3GPP-WLAN interworking        should not result in decreased but preferable in better user        experience).    -   Solutions should improve the utilization of the WLAN system when        it is available and not congested.    -   Solutions should reduce or maintain battery consumption (e.g.        due to WLAN scanning/discovery).    -   Solutions should be compatible with all existing CN WLAN related        functionality, e.g., seamless and non-seamless offload, trusted        and non-trusted access, multiple access connectivity (MAPCON)        and IP flow mobility (IFOM).    -   Solutions should be backward compatible with existing 3GPP and        WLAN specifications, i.e., work with legacy UEs even though        legacy UEs may not benefit from the improvements provided by        these solutions.    -   Solutions should rely on existing WLAN functionality and should        avoid changes to IEEE and Wi-Fi Alliance (WFA) specifications.

One of main purposes of the data offloading is to improve efficiency oflimited radio resources of the 3GPP system by using available WLANsystem. Accordingly, it is important to make full use of availableresources of the WLAN system, when exists.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for performing apartial handover procedure in a wireless communication system. Thepresent invention provides a method for offloading, by a user equipment(UE) connected to a 3rd generation partnership project (3GPP) system, apart of data to a wireless local area network (WLAN) system using apartial handover.

In an aspect, a method for performing, by an eNodeB (eNB) of a 3rdgeneration partnership project (3GPP) long term evolution (LTE) system,a partial handover procedure in a wireless communication system isprovided. The method includes determining E-UTRAN radio access bearers(E-RABs) to be offloaded to an access point (AP) of a wireless localarea network (WLAN) system, and transmitting an E-RAB release indicationmessage, which includes a list of the E-RABs to be offloaded to the APand a corresponding cause, to a mobility management entity (MME) of the3GPP LTE system.

The corresponding cause may be “Offload to WLAN”.

The E-RAB release indication message may further include a basic serviceset identifier (BSSID) of the AP.

The list of the E-RABs to be offloaded to the AP may correspond to anE-RAB Released List information element (IE) in the E-RAB releaseindication message.

The corresponding cause may correspond to a Radio Network Layer Cause IEin a Cause IE in an E-RAB List IE in the E-RAB release indicationmessage.

The E-RABs to be offloaded to the AP may be determined in order ofservices which are less influenced in spite of offloading to the AP.

The E-RABs to be offloaded to the AP may correspond to a non-real timeservice.

The method may further include receiving a measurement report includinga BSSID of the AP from a user equipment (UE).

The method may further include transmitting IDs of the E-RABs to beoffloaded to a UE via a radio resource control (RRC) connectionreconfiguration procedure.

In another aspect, a method for performing, by a mobility managemententity (MME) of a 3rd generation partnership project (3GPP) long termevolution (LTE) system, a partial handover procedure in a wirelesscommunication system is provided. The method includes receiving an E-RABrelease indication message, which includes a list of E-UTRAN radioaccess bearers (E-RABs) to be offloaded to an access point (AP) of awireless local area network (WLAN) system and a corresponding cause,from an eNodeB (eNB), and transmitting an E-RAB release request message,which includes the list of E-RABs to be offloaded to the AP and thecorresponding cause, to a packet data network (PDN) gateway (PDN-GW).

The E-RAB release request message may further include a BSSID of the AP.

In another aspect, a method for performing, by a packet data network(PDN) gateway (PDN-GW), a partial handover procedure in a wirelesscommunication system is provided. The method includes receiving an E-RABrelease request message, which includes a list of E-UTRAN radio accessbearers (E-RABs) to be offloaded to an access point (AP) of a wirelesslocal area network (WLAN) system and a corresponding cause, from amobility management entity (MME) of a 3rd generation partnership project(3GPP) long term evolution (LTE) system, and transmitting an offloadindication message, which includes an identifier (ID) of a userequipment (UE), to the AP.

Utilization of a WLAN system can be increased, and accordingly, anetwork and radio resources can be used effectively.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows LTE system architecture.

FIG. 2 shows a control plane of a radio interface protocol of an LTEsystem.

FIG. 3 shows a user plane of a radio interface protocol of an LTEsystem.

FIG. 4 shows an example of a physical channel structure.

FIGS. 5 and 6 show an intra-MME/S-GW handover procedure.

FIG. 7 shows an example of 3GPP-WLAN interworking architecture.

FIG. 8 shows an example of 3GPP-WLAN interworking architecture beforepartial handover and after partial handover according to an embodimentof the present invention.

FIG. 9 shows an example of a method for performing a partial handoverprocedure according to an embodiment of the present invention.

FIG. 10 shows a wireless communication system to implement an embodimentof the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

The technology described below can be used in various wirelesscommunication systems such as code division multiple access (CDMA),frequency division multiple access (FDMA), time division multiple access(TDMA), orthogonal frequency division multiple access (OFDMA), singlecarrier frequency division multiple access (SC-FDMA), etc. The CDMA canbe implemented with a radio technology such as universal terrestrialradio access (UTRA) or CDMA-2000. The TDMA can be implemented with aradio technology such as global system for mobile communications(GSM)/general packet ratio service (GPRS)/enhanced data rate for GSMevolution (EDGE). The OFDMA can be implemented with a radio technologysuch as institute of electrical and electronics engineers (IEEE) 802.11(Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802-20, evolved UTRA (E-UTRA), etc.IEEE 802.16m is an evolution of IEEE 802.16e, and provides backwardcompatibility with an IEEE 802.16-based system. The UTRA is a part of auniversal mobile telecommunication system (UMTS). 3rd generationpartnership project (3GPP) long term evolution (LTE) is a part of anevolved UMTS (E-UMTS) using the E-UTRA. The 3GPP LTE uses the OFDMA indownlink and uses the SC-FDMA in uplink. LTE-advance (LTE-A) is anevolution of the 3GPP LTE.

For clarity, the following description will focus on the LTE-A. However,technical features of the present invention are not limited thereto.

Handover (HO) is described. It may be referred to Section 10.1.2.1 of3GPP TS 36.300 V11.4.0 (2012-12).

The intra E-UTRAN HO of a UE in RRC_CONNECTED state is a UE-assistednetwork-controlled HO, with HO preparation signaling in E-UTRAN:

-   -   Part of the HO command comes from the target eNB and is        transparently forwarded to the UE by the source eNB;    -   To prepare the HO, the source eNB passes all necessary        information to the target eNB (e.g., E-UTRAN radio access bearer        (E-RAB) attributes and RRC context): When carrier aggregation        (CA) is configured and to enable secondary cell (SCell)        selection in the target eNB, the source eNB can provide in        decreasing order of radio quality a list of the best cells and        optionally measurement result of the cells.    -   Both the source eNB and UE keep some context (e.g., C-RNTI) to        enable the return of the UE in case of HO failure;    -   UE accesses the target cell via RACH following a contention-free        procedure using a dedicated RACH preamble or following a        contention-based procedure if dedicated RACH preambles are not        available: the UE uses the dedicated preamble until the handover        procedure is finished (successfully or unsuccessfully);    -   If the RACH procedure towards the target cell is not successful        within a certain time, the UE initiates radio link failure        recovery using the best cell;    -   No robust header compression (ROHC) context is transferred at        handover.

First, C-plane handling is described. The preparation and executionphase of the HO procedure is performed without EPC involvement, i.e.,preparation messages are directly exchanged between the eNBs. Therelease of the resources at the source side during the HO completionphase is triggered by the eNB. In case an RN is involved, its donor eNB(DeNB) relays the appropriate S1 messages between the RN and the MME(S1-based handover) and X2 messages between the RN and target eNB(X2-based handover); the DeNB is explicitly aware of a UE attached tothe RN due to the S1 proxy and X2 proxy functionality.

FIGS. 5 and 6 show an intra-MME/S-GW handover procedure.

0. The UE context within the source eNB contains information regardingroaming restrictions which were provided either at connectionestablishment or at the last TA update.

1. The source eNB configures the UE measurement procedures according tothe area restriction information. Measurements provided by the sourceeNB may assist the function controlling the UE's connection mobility.

2. The UE is triggered to send measurement reports by the rules set byi.e., system information, specification, etc.

3. The source eNB makes decision based on measurement reports and radioresource management (RRM) information to hand off the UE.

4. The source eNB issues a handover request message to the target eNBpassing necessary information to prepare the HO at the target side (UEX2 signalling context reference at source eNB, UE S1 EPC signallingcontext reference, target cell identifier (ID), K_(eNB*), RRC contextincluding the cell radio network temporary identifier (C-RNTI) of the UEin the source eNB, AS-configuration, E-RAB context and physical layer IDof the source cell+short MAC-I for possible radio link failure (RLF)recovery). UE X2/UE S1 signalling references enable the target eNB toaddress the source eNB and the EPC. The E-RAB context includes necessaryradio network layer (RNL) and transport network layer (TNL) addressinginformation, and quality of service (QoS) profiles of the E-RABs.

5. Admission Control may be performed by the target eNB dependent on thereceived E-RAB QoS information to increase the likelihood of asuccessful HO, if the resources can be granted by target eNB. The targeteNB configures the required resources according to the received E-RABQoS information and reserves a C-RNTI and optionally a RACH preamble.The AS-configuration to be used in the target cell can either bespecified independently (i.e., an “establishment”) or as a deltacompared to the AS-configuration used in the source cell (i.e., a“reconfiguration”).

6. The target eNB prepares HO with L1/L2 and sends the handover requestacknowledge to the source eNB. The handover request acknowledge messageincludes a transparent container to be sent to the UE as an RRC messageto perform the handover. The container includes a new C-RNTI, target eNBsecurity algorithm identifiers for the selected security algorithms, mayinclude a dedicated RACH preamble, and possibly some other parameters,i.e., access parameters, SIBs, etc. The handover request acknowledgemessage may also include RNL/TNL information for the forwarding tunnels,if necessary.

As soon as the source eNB receives the handover request acknowledge, oras soon as the transmission of the handover command is initiated in thedownlink, data forwarding may be initiated.

Steps 7 to 16 in FIGS. 5 and 6 provide means to avoid data loss duringHO.

7. The target eNB generates the RRC message to perform the handover,i.e., RRCConnectionReconfiguration message including themobilityControllnformation, to be sent by the source eNB towards the UE.The source eNB performs the necessary integrity protection and cipheringof the message. The UE receives the RRCConnectionReconfiguration messagewith necessary parameters (i.e. new C-RNTI, target eNB securityalgorithm identifiers, and optionally dedicated RACH preamble, targeteNB SIBs, etc.) and is commanded by the source eNB to perform the HO.The UE does not need to delay the handover execution for delivering theHARQ/ARQ responses to source eNB.

8. The source eNB sends the sequence number (SN) status transfer messageto the target eNB to convey the uplink PDCP SN receiver status and thedownlink PDCP SN transmitter status of E-RABs for which PDCP statuspreservation applies (i.e., for RLC AM). The uplink PDCP SN receiverstatus includes at least the PDCP SN of the first missing UL servicedata unit (SDU) and may include a bit map of the receive status of theout of sequence UL SDUs that the UE needs to retransmit in the targetcell, if there are any such SDUs. The downlink PDCP SN transmitterstatus indicates the next PDCP SN that the target eNB shall assign tonew SDUs, not having a PDCP SN yet. The source eNB may omit sending thismessage if none of the E-RABs of the UE shall be treated with PDCPstatus preservation.

9. After receiving the RRCConnectionReconfiguration message includingthe mobilityControllnformation, UE performs synchronization to targeteNB and accesses the target cell via RACH, following a contention-freeprocedure if a dedicated RACH preamble was indicated in themobilityControllnformation, or following a contention-based procedure ifno dedicated preamble was indicated. UE derives target eNB specific keysand configures the selected security algorithms to be used in the targetcell.

10. The target eNB responds with UL allocation and timing advance.

11. When the UE has successfully accessed the target cell, the UE sendsthe RRCConnectionReconfigurationComplete message (C-RNTI) to confirm thehandover, along with an uplink buffer status report, whenever possible,to the target eNB to indicate that the handover procedure is completedfor the UE. The target eNB verifies the C-RNTI sent in theRRCConnectionReconfigurationComplete message. The target eNB can nowbegin sending data to the UE.

12. The target eNB sends a path switch request message to MME to informthat the UE has changed cell.

13. The MME sends a modify bearer request message to the servinggateway.

14. The serving gateway switches the downlink data path to the targetside. The Serving gateway sends one or more “end marker” packets on theold path to the source eNB and then can release any U-plane/TNLresources towards the source eNB.

15. The serving gateway sends a modify bearer response message to MME.

16. The MME confirms the path switch request message with the pathswitch request acknowledge message.

17. By sending the UE context release message, the target eNB informssuccess of HO to source eNB and triggers the release of resources by thesource eNB. The target eNB sends this message after the path switchrequest acknowledge message is received from the MME.

18. Upon reception of the UE context release message, the source eNB canrelease radio and C-plane related resources associated to the UEcontext. Any ongoing data forwarding may continue.

FIG. 7 shows an example of 3GPP-WLAN interworking architecture. WLANnetwork described in FIG. 7 may be deployed and controlled by anoperator.

Referring to FIG. 7, a UE has a connection with a packet data network(PDN) gateway (PGW) through the eNB in the 3GPP system, and has aconnection with the PGW through the AP in the WLAN system. The AP andthe PGW may be connected through an evolved packet data gateway (ePDG).That is, the UE is connected to the eNB, the eNB may interwork with theAP.

When 3GPP-WLAN interworking is supported, a method for offloading datafrom the 3GPP system to the WLAN system when overload occurs at an eNBwhich serves a UE has been discussed. Hereinafter, a method foroffloading a part of data, used by the UE, from the 3GPP system to theWLAN system according to an embodiment of the present invention isdescribed, in order to make full use of available resources of the WLANsystem. According to the embodiment of the present invention, when a UE,served by an eNB of the 3GPP system, discovers an AP of the WLAN systemin the neighborhood, the UE may offload a part of received data to theWLAN system by using a partial handover procedure. Therefore, load ofthe 3GPP system can be reduced, and resource utilization of the WLANsystem can be improved.

FIG. 8 shows an example of 3GPP-WLAN interworking architecture beforepartial handover and after partial handover according to an embodimentof the present invention. FIG. 8-(a) shows a connected state of the UEwith the 3GPP system and the WLAN system before partial handover. Thereare three E-UTRAN radio access bearers (E-RABs) in the 3GPP system, andthere is one E-RAB in the WLAN system. FIG. 8-(b) shows a connectedstate of the UE with the 3GPP system and the WLAN system after partialhandover. One E-RAB among the three E-RABs in the 3GPP system isoffloaded to the WLAN system. Accordingly, overload in the 3GPP systemcan be reduced.

FIG. 9 shows an example of a method for performing a partial handoverprocedure according to an embodiment of the present invention.

A UE connected with the 3GPP system may scan an AP in the neighborhood.The AP may broadcast information on load of itself. Upon receiving thebroadcast information, the UE may acknowledge that some services, whichare being received through the 3GPP system, can be offloaded to the WLANsystem. Accordingly, in step S100, the UE transmits a measurement reportto the eNB in order to inform the eNB that there is an AP to which datacan be offloaded in the neighborhood. The measurement report may includea basic service set identifier (BSSID) of the AP.

Upon receiving the measurement report, in step S110, the eNB transmitsan E-RAB release indication message to the MME. The eNB may determineE-RABs to be released among E-RABs of the UE, and may inform the MME bytransmitting the e-RAB release indication message. The E-RABs to bereleased may be determined in order of services whose quality is lessinfluenced even though the service is received through the WLAN system.For example, non-real time service may be determined as E-RABs to bereleased first.

Table 1 shows an example of the e-RAB release indication messageaccording to an embodiment of the present invention. It may be referredto Section 9.1.3.7 of 3GPP TS 36.413 V11.2.0 (2012-12). The E-RABrelease indication message is transmitted by the eNB and is used toindicate the MME to release one or several E-RABs for one UE.

TABLE 1 IE type Seman- As- and tics Crit- signed IE/Group Pres- refer-descrip- ical- Critical- Name ence Range ence tion ity ity Message TypeM 9.2.1.1 YES ignore MME UE M 9.2.3.3 YES reject S1AP ID eNB UE S1AP IDM 9.2.3.4 YES reject E-RAB Released M E-RAB A value YES ignore List Listfor E- 9.2.1.36 RAB ID shall only be present once in E-RAB Released ListIE.

Referring to Table 1, the E-RAB release indication message includesE-RAB Released List information element (IE). The E-RAB Released List IEindicates a list of E-RABs of the UE to be released.

Table 2 shows an E-RAB list IE representing the E-RAB Released List IEincluded in the E-RAB release indication message according to anembodiment of the present invention. It may be referred to Section9.2.1.36 of 3GPP TS 36.413 V11.2.0 (2012-12). The E-RAB list IE containsa list of E-RAB IDs with a cause value. It is used for example toindicate failed bearers or bearers to be released.

TABLE 2 IE type Seman- As- and tics Crit- signed IE/Group Pres- refer-descrip- ical- Critical- Name ence Range ence tion ity ity E-RAB List 1. . . EACH ignore Item <maxnoofE- RABs> >E-RAB ID M 9.2.1.2 — — >Cause M9.2.1.3 — —

Table 3 shows a Cause IE included in the E-RAB list IE according to anembodiment of the present invention. It may be referred to Section9.2.1.3 of 3GPP TS 36.413 V11.2.0 (2012-12). The purpose of the Cause IEis to indicate the reason for a particular event for the S1AP protocol.

TABLE 3 IE/Group Semantics Name Presence Range IE Type and ReferenceDescription CHOICE M Cause Group >Radio Network Layer >>Radio MENUMERATED Network (Unspecified, Layer TX2_(RELOCOverall) Expiry, CauseSuccessful Handover, Release due to E-UTRAN Generated Reason, HandoverCancelled, Partial Handover, Handover Failure In Target EPC/eNB OrTarget System, Handover Target not allowed, TS1_(RELOCoverall) Expiry,TS1_(RELOCprep) Expiry, Cell not available, Unknown Target ID, No RadioResources Available in Target Cell, Unknown or already allocated MME UES1AP ID, Unknown or already allocated eNB UE S1AP ID, Unknown orinconsistent pair of UE S1AP ID, Handover desirable for radio reasons,Time critical handover, Resource optimisation handover, Reduce load inserving cell, User inactivity, Radio Connection With UE Lost, LoadBalancing TAU Required, CS Fallback Triggered, UE Not Available For PSService, Radio resources not available, Failure in the Radio InterfaceProcedure, Invalid QoS combination, Inter-RAT redirection, Interactionwith other procedure, Unknown E-RAB ID, Multiple E-RAB ID instances,Encryption and/or integrity protection algorithms not supported, S1intra system Handover triggered, S1 inter system Handover triggered, X2Handover triggered . . . , Redirection towards 1xRTT, Not supported QCIvalue, invalid CSG Id (Offload to WLAN(, WLAN ID (BSSID)) >TransportLayer >>Transport M ENUMERATED Layer (Transport Resource Unavailable,Cause Unspecified, . . . )

Referring to Table 3, the Cause IE includes a Radio Network Cause IE,which indicates various kinds of causes. According to an embodiment ofthe present invention, the Radio Network Cause IE may additionallyinclude “Offload to WLAN” cause in comparison with the conventionalRadio Network Cause IE. Accordingly, by the “Offload to WLAN” cause inthe Cause IE, it may be indicated that E-RABs of the UE are to bereleased due to offloading to the WLAN system. Further, the eNB mayinform the MME an ID of the AP, such as BSSID of the AP. For example,the Cause IE may include the BSSID of the AP. The MME may use the BSSIDof the AP in order to identify the AP.

Further, in step S140, the eNB informs the UE of an ID of a data radiobearer (DRB) to be released via an RRC connection reconfigurationprocedure.

Upon receiving the E-RAB release indication message, the MME mayinitiate an E-RAB release procedure in the core network. For this, instep S120, the MME transmits an E-RAB release request message to the PDNGW. The E-RAB release request message may include the “Offload to WLAN”cause. Further, the E-RAB release request message may include the BSSIDof the AP.

Upon receiving the E-RAB release request message, in step S130, the PDNGW transmits an offload indication message to the AP. By transmittingthe offload indication message, services corresponding to E-RABs to bereleased can be offloaded seamlessly to the WLAN system. The offloadindication message may include an ID of the UE. The offload indicationmessage may be transmitted to the AP via a direct message.Alternatively, if other nodes are deployed between the PDN GW and theAP, the offload indication message may be transmitted to the AP goingthrough the other nodes.

Upon receiving the offload indication message including the ID of theUE, the AP may associate with the corresponding UE with a higherpriority. Alternatively, the AP may associate with the corresponding UEaccording to a policy of the network.

FIG. 10 shows a wireless communication system to implement an embodimentof the present invention.

An eNB or MME or PDN-GW 800 includes a processor 810, a memory 820, anda radio frequency (RF) unit 830. The processor 810 may be configured toimplement proposed functions, procedures, and/or methods in thisdescription. Layers of the radio interface protocol may be implementedin the processor 810. The memory 820 is operatively coupled with theprocessor 810 and stores a variety of information to operate theprocessor 810. The RF unit 830 is operatively coupled with the processor810, and transmits and/or receives a radio signal.

A UE or eNB or MME or PDN-GW 900 may include a processor 910, a memory920 and a RF unit 930. The processor 910 may be configured to implementproposed functions, procedures and/or methods described in thisdescription. Layers of the radio interface protocol may be implementedin the processor 910. The memory 920 is operatively coupled with theprocessor 910 and stores a variety of information to operate theprocessor 910. The RF unit 930 is operatively coupled with the processor910, and transmits and/or receives a radio signal.

The processors 810, 910 may include application-specific integratedcircuit (ASIC), other chipset, logic circuit and/or data processingdevice. The memories 820, 920 may include read-only memory (ROM), randomaccess memory (RAM), flash memory, memory card, storage medium and/orother storage device. The RF units 830, 930 may include basebandcircuitry to process radio frequency signals. When the embodiments areimplemented in software, the techniques described herein can beimplemented with modules (e.g., procedures, functions, and so on) thatperform the functions described herein. The modules can be stored inmemories 820, 920 and executed by processors 810, 910. The memories 820,920 can be implemented within the processors 810, 910 or external to theprocessors 810, 910 in which case those can be communicatively coupledto the processors 810, 910 via various means as is known in the art.

In view of the exemplary systems described herein, methodologies thatmay be implemented in accordance with the disclosed subject matter havebeen described with reference to several flow diagrams. While forpurposed of simplicity, the methodologies are shown and described as aseries of steps or blocks, it is to be understood and appreciated thatthe claimed subject matter is not limited by the order of the steps orblocks, as some steps may occur in different orders or concurrently withother steps from what is depicted and described herein. Moreover, oneskilled in the art would understand that the steps illustrated in theflow diagram are not exclusive and other steps may be included or one ormore of the steps in the example flow diagram may be deleted withoutaffecting the scope and spirit of the present disclosure.

What is claimed is:
 1. A method for performing, by an eNodeB (eNB) of a3rd generation partnership project (3GPP) long term evolution (LTE)system, a partial handover procedure in a wireless communication system,the method comprising: determining E-UTRAN radio access bearers (E-RABs)to be offloaded to an access point (AP) of a wireless local area network(WLAN) system; and transmitting an E-RAB release indication message,which includes a list of the E-RABs to be offloaded to the AP and acorresponding cause, to a mobility management entity (MME) of the 3GPPLTE system.
 2. The method of claim 1, wherein the corresponding cause is“Offload to WLAN”.
 3. The method of claim 1, wherein the E-RAB releaseindication message further includes a basic service set identifier(BSSID) of the AP.
 4. The method of claim 1, wherein the list of theE-RABs to be offloaded to the AP corresponds to an E-RAB Released Listinformation element (IE) in the E-RAB release indication message.
 5. Themethod of claim 1, wherein the corresponding cause corresponds to aRadio Network Layer Cause IE in a Cause IE in an E-RAB List IE in theE-RAB release indication message.
 6. The method of claim 1, wherein theE-RABs to be offloaded to the AP are determined in order of serviceswhich are less influenced in spite of offloading to the AP.
 7. Themethod of claim 1, wherein the E-RABs to be offloaded to the APcorrespond to a non-real time service.
 8. The method of claim 1, furthercomprising: receiving a measurement report including a BSSID of the APfrom a user equipment (UE).
 9. The method of claim 1, furthercomprising: transmitting IDs of the E-RABs to be offloaded to a UE via aradio resource control (RRC) connection reconfiguration procedure.
 10. Amethod for performing, by a mobility management entity (MME) of a 3rdgeneration partnership project (3GPP) long term evolution (LTE) system,a partial handover procedure in a wireless communication system, themethod comprising: receiving an E-RAB release indication message, whichincludes a list of E-UTRAN radio access bearers (E-RABs) to be offloadedto an access point (AP) of a wireless local area network (WLAN) systemand a corresponding cause, from an eNodeB (eNB); and transmitting anE-RAB release request message, which includes the list of E-RABs to beoffloaded to the AP and the corresponding cause, to a packet datanetwork (PDN) gateway (PDN-GW).
 11. The method of claim 10, wherein thecorresponding cause is “Offload to WLAN”.
 12. The method of claim 11,wherein the E-RAB release indication message further includes a basicservice set identifier (BSSID) of the AP.
 13. The method of claim 11,wherein the E-RAB release request message further includes a BSSID ofthe AP.
 14. A method for performing, by a packet data network (PDN)gateway (PDN-GW), a partial handover procedure in a wirelesscommunication system, the method comprising: receiving an E-RAB releaserequest message, which includes a list of E-UTRAN radio access bearers(E-RABs) to be offloaded to an access point (AP) of a wireless localarea network (WLAN) system and a corresponding cause, from a mobilitymanagement entity (MME) of a 3rd generation partnership project (3GPP)long term evolution (LTE) system; and transmitting an offload indicationmessage, which includes an identifier (ID) of a user equipment (UE), tothe AP.
 15. The method of claim 14, wherein the corresponding cause is“Offload to WLAN”.